Eddy Current Probe (ECP) sensor systems have been used since the 1970's for non-contact displacement measurements, in the monitoring and protection of rotating machines mainly operating with journal (sleeve) bearings. ECP systems are also commonly known as “Proximity Probe Systems”.
Eddy current probes are dependent on a driver comprising an oscillator. The oscillator is used to excite the attached eddy current probe so that it can generate a changing magnetic field. This first magnetic field, when in close range to a steel target material, will induce time changing eddy currents in the surface of the target material. These eddy currents, in turn, will generate a second magnetic field that will oppose the originating first magnetic field and therefore affecting the resulting impedance of the probe tip. The size of the induced eddy currents is dependent on the distance between the probe tip and the steel target material. The probe impedance change is therefore a direct measurement of the distance between the probe tip and the target material.
Oscillator circuits in eddy current driver systems are commonly based upon a Collpits type oscillator using discrete matched transistor stages as active elements. The oscillator stage is current driven and basically operated in fully saturated mode, acting as a switch and thus providing the required energy to sustain an oscillation. The resulting amplitude is defined by the non-linearity of the drive currents and is temperature and device dependent as a result of parasitic influences. In order to attain a similar output response from multiple modules, this very low cost method requires amplitude calibration and also temperature and frequency compensation due to the used PN junctions of the driving transistors. In addition, amplitude stability is dependent on the stability of the load of the oscillator circuit due to output impedance and parasitic capacitors. Besides variations in parasitic influences between components, these capacitive load influences will also be frequency dependent and therefore affect the overall probe/cable tank impedance value. It is desirable that the oscillation frequency remains as constant as possible over the full operation range of a probe system even with various cable lengths. Furthermore, in order to tune the tank circuit for optimum impedance response for longer cable lengths, a parallel, commonly ferrite-based, load inductance is usually used in current systems. This ferrite-based inductor, however, will experience long-term change/drift over time and thus also have an effect on the tank circuit output impedance.
By using a constant drive current, the output voltage is a function of tank impedance and fundamental harmonic response of the excitation current. Due to these characteristics, for equal probe/cable systems, the resulting tank voltage is subject to level changes, and therefore cannot be assumed to be constant between different driver modules. This therefore requires undesirable manual calibration, and calls for better means of amplitude accuracy and stability. There thus seems to be room for improvement.